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Thread: Motoring questions you are afraid to ask.

  1. #11
    Senior Member Huggie's Avatar
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    Motoring questions you are afraid to ask.

    Quote Originally Posted by hi-rev
    Why do brake pedals turn soft over time, and have to be re-bled ?
    The system is closed, the fluid is the same, and the wear of the brakepad cannot be that great, so what makes the pedal soft ?
    There can be lots of stuff. As above, the fluid gets old, the flexi hoses can bulge a bit/go a bit elastic and if the pads and disks are in tatters there can be deformation under pressure and your piston/caliper may skew a bit etc etc.

    If the pedal is getting long as opposed to soft, then it can be a couple of other things. You can have contamination or leaks on seals or lines that let air into the system, hence needing a bleed. Worn wheel bearings will also give you a long pedal (too much travel before it firms-up). The slackness in the bearings will allow the brake disk will tilt slightly under the weight of the car pushing the pads/pistons back, so the first bit of pedal movement just closes up all the gaps and stands the disk back up. Two quick taps followed by a good push on the pedal sort of confirms this one but not conclusively.

    Strangely when I autotested a mini, the pedal used to go long but for a different reason. The car did so few miles and the pads wore so slowly that the caliper pistons and seals would stick together. I would occasionally have to remove the pads and pump out the pistons a bit just to get things moving but only push the piston back enough to wedge the pad back in. The seal sticking to the piston would deform as the piston moved out a of the caliper by a fraction of a millimetre but being stuck it would retract the piston once the brake pedal pressure went.

    An experienced hand can normally spot the problem quickly enough.

    H
    The noisy one in the corner.

  2. #12
    Senior Member Huggie's Avatar
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    Motoring questions you are afraid to ask.

    Quote Originally Posted by sleepy-vtec
    Quote Originally Posted by Ryan
    Why is reverse a straight cut gear?

    I know why forward gears are helical (most of the time) but can't figure why reverse isn't.
    I'm going to give an idiot answer here, that its so you can't select reverse instead of fourth when you're barreling along. This was one of my greatest fears when learning to drive!!!

    Firstly reverse gears are often straight cut because it is cheaper and easier to cut the teeth. They do not need to carry the power or sustain the wear that the forward gears do so don't need to be so refined (helically cut).

    The forward gears in your car use a "synchromesh" system that equalises the speeds of the gears that spin freely on the main shaft with the synchro-hubs that carry the power onto the main shaft. As you engage a gear, a tapered cone is pressed into a tapered ring which brings the two components to the same speed using the friction of your gear oil. Once the parts are close enough in speed, the oil film thickness is squeezed thinner and thinner until the gear parts can engage a ring of teeth on their mating faces. Put a lot more simply, your reverse gear generally has no synchromesh though I believe my Land Cruisers had a syncrho on reverse.

    Brain hurts now, I'm off to bed


    H
    The noisy one in the corner.

  3. #13

    Motoring questions you are afraid to ask.

    Terrific info and service as always Huggie, you're a credit to oct... Sorry 40aoY.

  4. #14

    Motoring questions you are afraid to ask.

    Informative post on the gears Huggie. As an additional point, because of their shape, the teeth in straight cut gears are generally stronger, but also noisier in use due to the way the face of the teeth engage - hence the pronounced whine in cars when you reverse.


    Quote Originally Posted by natnif
    Quote Originally Posted by hi-rev
    Why do saloon cars not get rear wipers? Hatchbacks have a steeper rake on the rear screen so their run off is better, but they still get one!
    The boot on saloons acts as a massive splitter, which helps stop the low level flows from ground level recirculating back up to the window. The water/dirt particles thrown up from the road only get as far as the vertical panel at the rear of a saloon car, and don't generally have enough energy to travel a further 2- 3ft upstream against the flow coming off the roof to reach the back window. Look at the dirt pattern on the back of any saloon. Hatchbacks just have one flat surface at the rear, with nothing to stop the dirty (in every sense) air from swirling round and coating the entire rear panel and window.

  5. #15
    Senior Member Huggie's Avatar
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    Motoring questions you are afraid to ask.

    [quote=-alan-]Informative post on the gears Huggie. As an additional point, because of their shape, the teeth in straight cut gears are generally stronger, but also noisier in use due to the way the face of the teeth engage - hence the pronounced whine in cars when you reverse.


    Quote Originally Posted by natnif
    Quote Originally Posted by "hi-rev":p2t6t0oc
    Why do saloon cars not get rear wipers? Hatchbacks have a steeper rake on the rear screen so their run off is better, but they still get one!
    The boot on saloons acts as a massive splitter, which helps stop the low level flows from ground level recirculating back up to the window. The water/dirt particles thrown up from the road only get as far as the vertical panel at the rear of a saloon car, and don't generally have enough energy to travel a further 2- 3ft upstream against the flow coming off the roof to reach the back window. Look at the dirt pattern on the back of any saloon. Hatchbacks just have one flat surface at the rear, with nothing to stop the dirty (in every sense) air from swirling round and coating the entire rear panel and window.[/quote:p2t6t0oc]

    Thanks for the help there -alan-. To show my appreciation I will add to your saloon/hatchback wiper answer too. The boot on a saloon also acts to streamline the air as it passes over this part of the car. By this, I mean it helps to prevent the air passing over the roof and rear screen from breaking out of a laminar (streamlined) pattern and into a turbulent pattern. This smooth streamlined air is separated from and sort of lubricated by the boundry layer in contact with the rear screen. This largely allows the contamination to stay in the air as it passes over the screen.

    On my E36 which I never wash enough, there is a distinct pattern of spray on the rear screen where the air passing along the side windows swirls over the rear quarter panels and meets the air passing over the roof and rear screen. One look at the rear spoiler on an E36 M3 shows that the design engineers in BMW catered for these two different air-flows in the design of the spoiler which follows the pattern of soil on the car. Look at the angle on the top surface in the centre and the underside towards each side of the spoiler below:




    Hatchbacks on the other hand have a big empty bubble of very turbulent air following them down the road which as you rightly point out can't support the mass or inertia of the muck stirred into it so it lands on the rear screen.

    There are drawbacks to being able to touch type at speed.... sorry!

    H
    The noisy one in the corner.

  6. #16

    Motoring questions you are afraid to ask.

    Quote Originally Posted by Wheelnut
    How do you re-size pictures so that they fit in the avatars here? (I think I used to know but I forget)
    Wheelnut,

    If you are using XP google search for and download the Image resizer Powertoy (on Vista/Win 7/Win 8) search for VSO Image Resizer.

    Once installed, you can right click on any image in Windows explorer and you'll have a new 'Resize' option.

    In here you can either set the 90x90 pixel size and click create/go/okay (I can't recall), or in the VSO/New version create a profile called Avatar and set it to 90 x 90 pixels.

  7. #17

    Motoring questions you are afraid to ask.

    Quote Originally Posted by br1an
    Quote Originally Posted by Wheelnut
    How do you re-size pictures so that they fit in the avatars here? (I think I used to know but I forget)
    Wheelnut,

    If you are using XP google search for and download the Image resizer Powertoy ...
    Thanks Br1an, I'm feeling a bit stupid now. I downloaded that ap about a year ago and expected to find it on the desktop. It wasn't there so I downloaded it again recently but it still wasn't there! So you tell me to right click on the picture and there it is!
    ops:
    Now what do I do if the picture is already on Photobucket?
    ________
    Wheelnut

  8. #18
    Senior Member Huggie's Avatar
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    Motoring questions you are afraid to ask.

    Quote Originally Posted by Wheelnut
    Quote Originally Posted by br1an
    Quote Originally Posted by Wheelnut
    How do you re-size pictures so that they fit in the avatars here? (I think I used to know but I forget)
    Wheelnut,

    If you are using XP google search for and download the Image resizer Powertoy ...
    Thanks Br1an, I'm feeling a bit stupid now. I downloaded that ap about a year ago and expected to find it on the desktop. It wasn't there so I downloaded it again recently but it still wasn't there! So you tell me to right click on the picture and there it is!
    ops:
    Now what do I do if the picture is already on Photobucket?
    • I would say:
    • Right click the picture on photobucket and save as "something like my_avatar.jpg" in some place you can find like your desktop.[/*:m:uqrfwenw]
    • Do your resize on the image while it is on your machine. 90x90 is good[/*:m:uqrfwenw]
    • Go to your user control panel on 40AOY.freeforum etc[/*:m:uqrfwenw]
    • Find the Edit Avatar on your Profile tab[/*:m:uqrfwenw]
    • Click on Choose File[/*:m:uqrfwenw]
    • Click Submit[/*:m:uqrfwenw]


    Am I close?


    H
    The noisy one in the corner.

  9. #19

    Motoring questions you are afraid to ask.

    Open the picture in paint and click to resize it unitl it comes down to 100pixels (ridiculously tiny IMO!)

    Disclaimer : The above post is my own PERSONAL OPINION and DOES NOT REPRESENT the official position of any company or entity. It DOES NOT constitute mechanical advice

  10. #20

    Motoring questions you are afraid to ask.

    Or post the pic in here and I'll do it for you

    Disclaimer : The above post is my own PERSONAL OPINION and DOES NOT REPRESENT the official position of any company or entity. It DOES NOT constitute mechanical advice

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